1. Field of the Invention
The invention relates to an axial turbine of an exhaust-gas turbocharger connected to an internal combustion engine according.
2. Discussion of Background
A decisive criterion for increasing the output of internal combustion engines connected to exhaust-gas turbochargers is the boost pressure applied by the compressor of the exhaust-gas turbocharger. If the boost pressure is increased, more air can be forced into the cylinders and the output of the internal combustion engine can thus be improved. In order to achieve high boost pressures, the exhaust-gas turbochargers used today rotate at very high circumferential velocities. The result of this, in particular in the case of relatively large exhaust-gas turbochargers, is that the fragments of a ruptured moving blade can only be retained in the turbine casing by elaborate design measures. As a result of the relatively large mass of the possible fragments, this problem is further increased in the so-called integral turbines, since their turbine disks and moving blades are made in one piece.
In the known exhaust-gas turbochargers having an axial turbine, the turbine disk is disposed axially in the gas-outlet casing and its moving blades are bounded radially to the outside by a cover ring/diffuser. In the extreme case, the turbine disk of the axial turbine is arranged in the middle of the gas-outlet casing (see article by M. Appel et al. on the subject "Turbolader hoher spezifischer Leistung . . . " Turbochargers of high specific output . . . !, in MTZ 54(1993)6, FIG. page 288). Since in this solution the outer wall of the gas-outlet casing or the flue follows directly after the thin cover ring/diffuser in the radial direction, virtually no resistance is offered to the fragments thrown outward at high speed if a moving blade or the turbine disk ruptures. Therefore the outer wall of the turbocharger may be pierced and thus persons may be endangered or adjacent machine parts damaged.
In order to prevent this, the exhaust-gas turbochargers are often provided with external rupture protection. However, such cladding attached to the outer wall of the axial turbine is very elaborate and thus expensive. DE-A1-42 23 496 also discloses internal rupture protection for an axial turbine. To this end, a protective ring extending axially in the region of the turbine disk is fastened to the turbine casing. This protective ring is arranged radially between the casing wall and the turbine disk at a slight distance from its rotational plane. However, apart from the assembly effort, such a separate rupture ring also requires additional production costs, which in turn increases the overall costs of the exhaust-gas turbocharger.
In addition, the MAN B&W NA turbocharger series (company brochure D366002/2E "NA-type turbochargers with axial-flow turbines", page 5, FIG. 4) discloses a rupture-protection ring designed as an integral, essentially radially extending, part of the gas-inlet-side wall of the gas-outlet casing. Said rupture-protection ring is connected to both the gas-inlet casing and the gas-outlet casing via an axial extension piece arranged downstream. To this end, a complicated, i.e. elaborate, connection between the extension piece and the gas-inlet casing is realized by means of relatively long flanges. However, such connecting elements arranged in direct proximity to the hot exhaust gases of the internal combustion engine which flow through the turbine are subjected to high thermal stresses and are therefore at great risk of breaking off. As a result, the functional reliability of such an exhaust-gas turbocharger may be put at risk and its service life reduced.